The invention disclosed and claimed herein is generally directed to a technique for acquiring magnetic resonance (MR) signal data relating to the flow of blood or other fluid within an imaging subject, wherein flow velocity or other parameter is encoded in the MR signals. More particularly, the invention pertains to a technique of the above type, wherein flow velocity information is encoded in the magnitude as well as the phase of respective MR signals. As is well known, acquired MR imaging data can be represented in quadrature. That is, the MR signal at each voxel location of an imaging slice can be represented by two complex components which are ninety degrees out of phase, referred to herein as C.sub.1 and C.sub.2, respectively. Thus, the MR signal at the jth voxel has a magnitude I.sub.j equal to ##EQU1## and a phase component .phi..sub.j equal to the arctangent of (C.sub.2 /C.sub.1), or tan.sup.-1 (C.sub.2 /C.sub.1). Accordingly S.sub.j can be expressed as S.sub.j =I.sub.j e.sup.-i.PHI..sbsp.j.
MR signal phase component is now used in the field of phase contrast (PC) angiography. Such field pertains to generation of an image which represents the flow of blood or other fluid through a vessel, and more specifically represents fluid velocity, rate of flow, or other flow parameter. As is further well known, if blood is flowing through the jth voxel, an associated signal S.sub.j can be acquired wherein the phase .phi..sub.j is a function of flow velocity at the jth voxel v.sub.j. Thus, velocity is encoded in the phase, i.e., .phi..sub.j =.PHI.(v.sub.j).
In accordance with conventional PC angiography, two consecutive MR experiments or sequences are applied to an imaging volume to produce two successive signals S.sub.1j and S.sub.2j, associated with the jth voxel. The first experiment employs a positive flow encoding gradient pulse, and the second experiment employs an identical gradient pulse, but of negative sign. Thus, signals are provided wherein S.sub.1j =I.sub.js +I.sub.1jm e.sup.-i.PHI.(vj) and S.sub.2j =I.sub.2js +I.sub.2jm e.sup.+i.PHI.(v.sub.j). The terms I.sub.1js and I.sub.2j represent the signal components resulting from static or non-moving spins and are the same for both experiments, i.e., I.sub.1js =I.sub.2js. Also, in conventional PC angiography, signal magnitude is not affected by flow velocity, so that I.sub.1jm =I.sub.2jm =I.sub.j. Accordingly, the difference between S.sub.1j and S.sub.2j, i.e. {I.sub.1js +I.sub.j e.sup.-.PHI.(v.sbsp.j) }-{I.sub.2js +I.sub.j e.sup.+.PHI.(v.sbsp.j) }=I.sub.j {e.sup.-.PHI.(v.sbsp.j) -e.sup.+l.PHI.(v.sbsp.j) },provides an expression from which static effects are eliminated, and from which the flow velocity may be computed.
Flow velocity v.sub.j generally comprises components v.sub.x, v.sub.y, and v.sub.Z, relative to the X-, Y-, Z- gradient axes, respectively. Practical implementation of the above technique requires computation of each velocity component, so that it is necessary to apply X-, Y-, and Z-gradients sequentially. As a result, the imaging time required to construct an angiogram is comparatively long. Under conventional practice, it is necessary to acquire at least four images, in order to obtain the velocity component information required to compute the velocity v.sub.j value for each of the voxels of a single angiogram. Imaging time thus constitutes one of the major limitations of PC angiography.